Hello @iacovoni

To disable the header image in smartphones and tablets you need to add custom CSS.
For this you need to go to Admin Panel / Appearance / Customzie / Theme Options / Advanced Options.
Here you need to copy and paste below CSS:

@media (max-width: 1023px) {
	header#masthead{
  		background-image: none;
 	}
}

Hope this will help to resolve your query.
Let us know how it goes.

Regards!!!